Half-billion-year-old fossil of strange Cambrian creature gives evolutionary clues about vertebrates

A stunningly-preserved, half-billion-year-old fossil of a strange Cambrian creature could shift our understanding of how a sister group to vertebrates evolved, a new study suggests.

The fossil, first reported on by The journal Nature Communications, is the earliest of its kind and belonged to the oldest tunicate species, Megasiphon thylakos. The fossil’s discovery has the potential to answer a crucial question regarding what the first tunicates were like and fills a gap in the tree of life of the animal kingdom.

The fossil that was recently discovered offers the “best possible window into what early tunicates looked like, what they lived like, what their ecology was, their lifestyle, and that in turn lets us make some inferences about what early tunicates and early vertebrates may have shared in common,” coauthor of the study Karma Nanglu who is an assistant professor in the Museum of Comparative Zoology at Harvard University, told Live Science.

Tunicates are sister groups of vertebrates, meaning they shared a common ancestor hundreds of millions of years ago. They’re a diversified subphylum of marine animals and invertebrates encompassing 3,000 modern species. They are classified into two distinct groups: appendicularians and ascidiacea. They are sometimes referred to as “sea squirts,” ascidians begin their lives in a tadpole-like shape and permanently cling to the sea floor until they transform into barrel-like adults. Appendicularians keep their tadpole form until adulthood and continue to swim in the ocean column. It was not known if the earliest tunicates resembled appendicularians or ascidians -which is crucial in understanding the evolutionary origins of vertebrates because of the close relationship between tunicates and ascidians.

We don’t know much about the ancient tunicates since we only have one find from their initial development period. A strange animal that isn’t as similar to modern tunicates is called Shankouclava, discovered over two decades ago in China. Fossils are scarce because tunicates were scarce, or could be due to their soft bodies not lasting very well. Nanglu said.

Evolutionary clues

The study’s coauthors, Javier Ortega-Hernandez and Rudy Lerosey-Aubril, who is an assistant professor of evolutionary and organismic biological sciences and researcher both at Harvard University, stumbled upon the fossil that was recently described within a collection of fossils in the Utah Museum of Natural History. Nanglu identified the distinctive tunicate features, including a barrel-shaped body and siphon-like growths. But he also noted the most notable feature, the dark bands running throughout the body.

The team examined the fossil using microscopy and high-resolution photographs and studied dissections of the fossil with the modern form of ascidiacea. The dissections proved that the similarity in both species’ structure was not only present at the surface. The dark bands reflected muscle arrangements akin to those of M. Thylakos and its contemporary counterpart.

“What this tunicate tells us is that tunicates ancestrally were most likely to be animals that had a lava form like a tadpole and then metamorphosed into a barrel-shaped animal that had these two siphons pointing up into the water column,” Nanglu explained.

This two-phase existence is believed to date back to the time of the tunicates’ origins. The results also place the date of birth for tunicates at the time of 500 million years, which is not that far from estimations made using “molecular clocks,” or mutation rates of DNA, the study’s authors write in their research.

The fossil’s identification found in the middle of the Cambrian Marjum Formation in Utah can be used to answer the issue of whether ascidian or appendicularians were the first to take a branch from this tunicate life tree. The close resemblance between M. Thylakos and ascidiaceans indicates that ascidiacea’s form is ancient, a notion that has been confirmed by research conducted in recent years.

In the next phase, Nanglu hopes to discover fossils that could show the time that deuterostomes intersect chordates (including vertebrates and tunicates) as well as Hemichordates (including marine creatures that resemble worms) and Echinoderms (such as sea cucumbers) to discover the link that connects all of them within an evolutionary context.

“We’re still searching for other animals that fit into this tree of life,” Nanglu said. Nanglu.


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